Ground Penetrating Radar cost?

[TRX]

I took out a gigondomundous tree when I built shop 30-odd years ago. The spot where the tree was is more or less smack in the middle of the concrete slab.

Now I'd like to put a lift in... right about... there.

We've had termite troubles off and on for years; I have a suspicion that where the lower trunk and roots of that tree were are mostly termite droppings now.

The obvious solution is to have someone come out and run a ground penetrating radar check. But the phone book and web aren't showing any local companies that do this; just some "consulting engineer" firms with out-of-state addresses, who sound like they'd be delighted to help you with your new shopping mall or nuclear power plant.

For those of you who've had GPR done, what did it cost you? I'd like to have some idea of general pricing before I "request an engineer come out to make an evaluation", as several of the sites liked to phrase it...

 

[billspit]

What are you hoping to find out with the GPR? The company I used to work for often hired locating companies that had a GPR. We were looking for underground tanks, pipelines septic tanks etc. The screen on the monitor is almost incomprehensible to anybody but the expert. BTW we usually paid $500-750 for the basic site.

 

[strutaeng]

I took out a gigondomundous tree when I built shop 30-odd years ago. The spot where the tree was is more or less smack in the middle of the concrete slab.

Now I'd like to put a lift in... right about... there.

We've had termite troubles off and on for years; I have a suspicion that where the lower trunk and roots of that tree were are mostly termite droppings now.

The obvious solution is to have someone come out and run a ground penetrating radar check. But the phone book and web aren't showing any local companies that do this; just some "consulting engineer" firms with out-of-state addresses, who sound like they'd be delighted to help you with your new shopping mall or nuclear power plant.

For those of you who've had GPR done, what did it cost you? I'd like to have some idea of general pricing before I "request an engineer come out to make an evaluation", as several of the sites liked to phrase it...

You are trying to determine if you have a void under the slab? Did you take out the rootball 30 years ago?

Around here just to get a guy out to a job site it is about $800. Outside of the service area I believe they do mileage, lodging and meals. Probably $1500 for remote areas near a big city?

Maybe try these guys?

https://www.groundpenetratingradararkansas.com/

 

[Rory Bellows]

Why not skip the radar and just drill a hole and pump grout or something to fill the void. After all, you will have to that anyway if there is a large void.

 

[firebirdparts]

I was thinking that too. The fix maybe is cheaper than finding out if you need it.

 

[Kaizen]

Ditto. The radar will show variations in material but not sure if it will show a void. How far down did you go when you took it out?


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[rburke65]

Just cut out the area of the post and you’ll know with what you’re dealing.

 

[johnnyradiant]

Pretend you only have a 2" slab where you want your hoist. Cut out the area and make sure you have something to call a base and fill the area back up in the best manner to accommodate your hoist. I'm betting if you don't actually open it up you might never have peace over the installation anyway. Termites or not I'd always have that dam stump on my mind if it were under my shop.

If the radar showed it to be there would it show it's strength of just that an object of different density than the surrounding base or earth was there? If it was guessed to be 'ok' to install what about another 5 yrs or more out?

Or are you hoping to see if it is/was not right where you need to put the hoist?

 

[flat350]

Call a place that does core drilling or slab sawing,they usually have a guy or know someone who will scan the slab.

 

[rustyjames]

I would just saw cut the slab to required dimensions, dig it out and refill with suitable material. Compacted accordingly.

 

[CraigStu]

I would just saw cut the slab to required dimensions, dig it out and refill with suitable material. Compacted accordingly.

I agree. Based on several exposed tree trunks that were cut down to about ground level and left there. Apparently the roots slowly but surely rot away because I have to keep adding gravel just to maintain the surface level enough to mow. This is on an extended area of our lot that is far enough from the house I don't really care how it looks. I just need to keep the garden tractor wheels from dropping into soft spots. So I can only imagine what is under your slab.

 

[curiousB]

Not sure what you are wanting to fix? Lots of prescriptions but no identified problem.

Is the floor shifting or cracking? Is it sinking or heaving in some way?

You said you pored it 30 years ago, how thick did you pour the slab? What reinforcement was used in the concrete?

I would say if the floor is in good shape and not showing signs of problems and it meets the thickness criteria for your lift I would do precisely nothing. Just install the lift and move forward.

 

[pistolpete1313]

I think he just wants to make sure he's not putting his lift over a cavity

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[ItsNemo]

I think he just wants to make sure he's not putting his lift over a cavity

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Sure, in which case it's easier/cheaper to just core the slab and patch than it is to do some fancy radar thing for $1000+.

 

[Jrad235]

A mudjacking company can come out and drill some holes, tell you if there is a void there, then pump it full of grout to stabilize it, probably altogether for less than $800. If just drilling and patching holes with no void, $1-300.

 

[rlitman]

A mudjacking company can come out and drill some holes, tell you if there is a void there, then pump it full of grout to stabilize it, probably altogether for less than $800. If just drilling and patching holes with no void, $1-300.

Yes, however, if there is a large mass of organic material rotting beneath the slab, filling it with grout is just a stopgap measure, as the remains will continue to rot.

 

[Bigblockyeti]

I was thinking that too. The fix maybe is cheaper than finding out if you need it.

+1, kinda like throwing ten $20 parts at a problem vs. paying someone $300 to determine exactly which $20 part needs to be replaced.

 

[Handyfarmer]

one can drill a hole 1/2" or 5/8" clear through the slab, and take a 3/8 rod, and just push it into the hole may need to weld a tee handle on it, to have a way to push on it, do it in a few places, most likely if there a void under the slab you will find it,

 

[Abeo]

If the concern is about having a proper footing for the lift, I'd skip the diagnostics/inferred data and just assume there is a void, and put in a proper footer (as prescribed by the lift manufacturer)

Something like a 3ft x 3ft x 10ft block. Digging down that far will tell you what's going on with the tree as well, and there will be no questions about whether there is a void or soft spots or whatnot.

 

[Samh]

First thing I would do would be to drag a chain over the area you are thinking about putting the lift. That is a non-destructive way to test concrete. And depending on the results would depend on whether or not I go any further with testing.

 

[mike93lx]

First thing I would do would be to drag a chain over the area you are thinking about putting the lift. That is a non-destructive way to test concrete. And depending on the results would depend on whether or not I go any further with testing.

Huh?

 

[Fix Until Broke]

Huh?

It'll sound different when you drag a log chain over concrete that has a hollow spot under it vs solid.

TRX - I'd guess that after 30 years, termites or not, it's stabilized, rotted, decayed, etc.

Assuming that your slab meets the lift manufacturers requirements, I'd mark and drill the holes for the hoist in your existing slab. Drill the holes all the way through and do what Handyfarmer suggested - use a rod with a T handle on it like they use to search for people in avalanches.

If everything is solid in the 10+ holes that you need to mount a hoist, I'd say you're good to go - bolt it in and move on.

If you strike "soft and hollow" then you've got a few options:
1) Call the mudjackers in and have them pump it full
2) Cut out the slab, excavate and put in proper fill
3) Probably some other ideas?

Either way, nothing to loose by drilling the holes that you'll need anyway.

 

[Samh]

Huh?

Probably should have elaborated as Fix Until Broke said, it will sound different when drug over a void or unsound concrete.

 

[TRX]

one can drill a hole 1/2" or 5/8" clear through the slab, ... most likely if there a void under the slab you will find it,

I think that suggestion is a winner. And it gives me an excuse to buy a hammer drill...

 

[curiousB]

And why is a void a problem if the slab is continuous and >4" thick?

What about a lift put on the second floor of a concrete building. There is a void.....

 

[mike93lx]

I think that suggestion is a winner. And it gives me an excuse to buy a hammer drill...

Rotary hammer....

 

[rlitman]

And why is a void a problem if the slab is continuous and >4" thick?

What about a lift put on the second floor of a concrete building. There is a void.....

But that's on a thicker slab that has more reinforcement.

 

[curiousB]

But that's on a thicker slab that has more reinforcement.

Yes but the bending moment of the baseplate, especially under load, is putting tension on all the fasteners, not compression. Those force vectors are upwards. As such having fill under the slab seems moot.

 

[TRX]

Rotary hammer....

Thanks for mentioning that... I just got a crash course on the difference between a "hammer drill" and a "rotary hammer drill."

 

[rlitman]

Yes but the bending moment of the baseplate, especially under load, is putting tension on all the fasteners, not compression. Those force vectors are upwards. As such having fill under the slab seems moot.

Huh, no. HALF the fasteners may be under tension. The other half will be under compression. Since the sum of the forces must be zero, the compression forces will be equal to the weight of the vehicle plus the weight of the lift PLUS the total tension.

 

[Abeo]

Also, having an unsupported span under that load means it puts the concrete into tension (from trying to sag). Concrete doesn't fail under compression, it fails under tension.

 

[rlitman]

Also, having an unsupported span under that load means it puts the concrete into tension (from trying to sag). Concrete doesn't fail under compression, it fails under tension.

The underside will be under increased tension. That's why parking garages have tendon wires in their slabs.

 

[Abeo]

The underside will be under increased tension. That's why parking garages have tendon wires in their slabs.

Are you talking about post-tension wires? Those are to keep the internal stresses of the concrete such that the concrete doesn't see tension. It puts it in a high state of compression with the goal of the compressive stresses reaching 0 at their lowest (with safety factors and loading factors included).

Edit: Yes, the underside will be under tension. Correct.

 

[rlitman]

Are you talking about post-tension wires? Those are to keep the internal stresses of the concrete such that the concrete doesn't see tension. It puts it in a high state of compression with the goal of the compressive stresses reaching 0 at their lowest (with safety factors and loading factors included).

Edit: Yes, the underside will be under tension. Correct.

Pre or post tension. Either way, tensioning the wires allows for a thinner concrete deck over wider spans, and the rotten void under this slab is essentially a span. But of course, it's not at all relevant here, since re-casting the slab to add more steel without filling the void would be insane.

 

[Abeo]

Pre or post tension. Either way, tensioning the wires allows for a thinner concrete deck over wider spans, and the rotten void under this slab is essentially a span. But of course, it's not at all relevant here, since re-casting the slab to add more steel without filling the void would be insane.

 

[rustyjames]

I think dragging the chain, and drilling/poking/probing are good ideas to get a quick reference on what's going on underneath the slab.

 

[bczygan]

You'll be fine.........!

Bill

 

[bczygan]

If the concern is about having a proper footing for the lift, I'd skip the diagnostics/inferred data and just assume there is a void, and put in a proper footer (as prescribed by the lift manufacturer)

Something like a 3ft x 3ft x 10ft block. Digging down that far will tell you what's going on with the tree as well, and there will be no questions about whether there is a void or soft spots or whatnot.

This! It would give me the most peace of mind.

This is better than mud jacking. Mud jacking doesn't compact the loose fill where the tree roots have rotted.

I might bore a series of 3 or 4" cores and check for voids and see what the compaction is, first.

When placing the new foundation, be sure to tie it into the existing slab.

Bill

 

[TRX]

What would be an economical way to cut some 3 or 4 inch holes in the slab?

For that matter, it would be nice if I could cut some holes big enough to reach through with a post hole digger. I've dug enough post holes near the shop I know what the soil *ought* to look like.

 

[Fix Until Broke]

You can rent core drills at Home Depot - probably the least expensive. You can get them for a rotary hammer. I've done it with my SDS+ at 4" - not rated for it, but if you take it slow it's fine. I wasn't going to buy a SDS Max just for this one size...

As I mentioned before, just start with anchor holes for the hoist and drill them through. Use a probe to gauge if the soil is firm or not. You can use the same probe somewhere that you're confident the soil is good for a comparison. My guess is that it will be obvious weather or not it's solid or not.

Were you able to drag a chain or something similar over the area as a simple check? Even a big bolt on a string will sound different.